摘要:

Comparative neuroanatomists since Herrick [1914] have been aware of the paradox that the brain of amphibians, especially salamanders, is less complex than one would expect based on their phylogenetic position among the Tetrapoda. Many features of the brain are less differentiated in salamanders than in tetrapod outgroups, including chondrichthyans and bony fishes, and for some brain characters, the salamander brain is even more simple than that of the agnathans. Here, we perform a cladistic analysis on 23 characters of four sensory systems (visual, auditory, lateral line and olfactory) and the brain. Our taxa include myxinoids, lampreys, chondrichthyans, actinopterygians, Latimeria, Neoceratodus and the Iepidosirenid lungfishes, amniotes, frogs, caecilians, salamanders and bolitoglossine salamanders. Of the 23 characters we examined, 19 are most parsimoniously interpreted as secondarily simplified in salamanders from a more complex ancestral state, two characters are equally parsimonious under both hypotheses, one character (well developed ipsilateral retinotectal projections) is more complex in bolitoglossine salamanders than in vertebrates generally, and only one character (migration of neurons in the medial pallium) is most parsimoniously interpreted as retention of the plesiomorphically simple condition. Secondary simplification of the salamander brain appears to result from paedomorphosis, or retention of juvenile or embryonic morphology into adulthood. Paedomorphosis is correlated with an increase in genome size, which in turn is positively correlated with cell size, but negatively correlated with cell proliferation and differentiation rates. Available data suggest that, although increasing genome size and paedomorphosis tend to compromise the function of the salamander brain, compensating mechanisms have evolved that may restore or even enhance brain function.

ISSN:0006-8977    

DOI:10.1159/000114147

出版商:S. Karger AG    

年代:1993

数据来源: Karger

摘要:

Comparative neuroanatomists since Herrick [1914] have been aware of the paradox that the brain of amphibians, especially salamanders, is less complex than one would expect based on their phylogenetic position among the Tetrapoda. Many features of the brain are less differentiated in salamanders than in tetrapod outgroups, including chondrichthyans and bony fishes, and for some brain characters, the salamander brain is even more simple than that of the agnathans. Here, we perform a cladistic analysis on 23 characters of four sensory systems (visual, auditory, lateral line and olfactory) and the brain. Our taxa include myxinoids, lampreys, chondrichthyans, actinopterygians, Latimeria, Neoceratodus and the Iepidosirenid lungfishes, amniotes, frogs, caecilians, salamanders and bolitoglossine salamanders. Of the 23 characters we examined, 19 are most parsimoniously interpreted as secondarily simplified in salamanders from a more complex ancestral state, two characters are equally parsimonious under both hypotheses, one character (well developed ipsilateral retinotectal projections) is more complex in bolitoglossine salamanders than in vertebrates generally, and only one character (migration of neurons in the medial pallium) is most parsimoniously interpreted as retention of the plesiomorphically simple condition. Secondary simplification of the salamander brain appears to result from paedomorphosis, or retention of juvenile or embryonic morphology into adulthood. Paedomorphosis is correlated with an increase in genome size, which in turn is positively correlated with cell size, but negatively correlated with cell proliferation and differentiation rates. Available data suggest that, although increasing genome size and paedomorphosis tend to compromise the function of the salamander brain, compensating mechanisms have evolved that may restore or even enhance brain function.

ISSN:0006-8977    

DOI:10.1159/000114149

出版商:S. Karger AG    

年代:1993

数据来源: Karger

摘要:

Comparative neuroanatomists since Herrick [1914] have been aware of the paradox that the brain of amphibians, especially salamanders, is less complex than one would expect based on their phylogenetic position among the Tetrapoda. Many features of the brain are less differentiated in salamanders than in tetrapod outgroups, including chondrichthyans and bony fishes, and for some brain characters, the salamander brain is even more simple than that of the agnathans. Here, we perform a cladistic analysis on 23 characters of four sensory systems (visual, auditory, lateral line and olfactory) and the brain. Our taxa include myxinoids, lampreys, chondrichthyans, actinopterygians, Latimeria, Neoceratodus and the Iepidosirenid lungfishes, amniotes, frogs, caecilians, salamanders and bolitoglossine salamanders. Of the 23 characters we examined, 19 are most parsimoniously interpreted as secondarily simplified in salamanders from a more complex ancestral state, two characters are equally parsimonious under both hypotheses, one character (well developed ipsilateral retinotectal projections) is more complex in bolitoglossine salamanders than in vertebrates generally, and only one character (migration of neurons in the medial pallium) is most parsimoniously interpreted as retention of the plesiomorphically simple condition. Secondary simplification of the salamander brain appears to result from paedomorphosis, or retention of juvenile or embryonic morphology into adulthood. Paedomorphosis is correlated with an increase in genome size, which in turn is positively correlated with cell size, but negatively correlated with cell proliferation and differentiation rates. Available data suggest that, although increasing genome size and paedomorphosis tend to compromise the function of the salamander brain, compensating mechanisms have evolved that may restore or even enhance brain function.

ISSN:0006-8977    

DOI:10.1159/000316117

出版商:S. Karger AG    

年代:1993

数据来源: Karger

摘要:

Specific binding sites for GABAA and benzodiazepines were detected in the brain of the frog Rana esculenta after the in vitro incubation of tissue sections with their respective specific agonists [3H] muscimol and [3H] flunitrazepam. Conditions for the binding assay were optimized and as a result binding was saturable and specific. Quantitative autoradiographic receptor measurements in the different brain sections showed that elevated levels of [3H] muscimol binding sites were found in the two layers of the cerebellum (periventricular and external) with the highest binding densities being detected in the periventricular layer. Relatively high densities of [3H] muscimol binding sites were also observed in the torus semicircularis of the mesencephalon and in the thalamic nucleus rotundus and posterolateral nucleus, plus the mitral cell layer of the olfactory bulb, the amygdala pars lateralis and the striatum of the telencephalon. Intermediate to low binding levels were obtained in the remaining brain areas such as the external layer of the optic tectum, the dorsomedial and dorsolateral anterior thalamic nuclei, the medial and lateral pallium, the medial septal nucleus, the preoptic nucleus, the dorsal and ventral infundibular nuclei of the hypothalamus and the interpeduncular nucleus. Autoradiographic evaluation of benzodiazepine receptors revealed that binding levels of [3H] flunitrazepam were overall lower than those of the GABAA sites. In fact the highest [3H] flunitrazepam binding levels were observed in the striatum, external layer of the optic tectum and the torus semicircularis. Intermediate receptor levels were encountered in the mitral cell and glomerular layers of the olfactory bulb, the nucleus rotundus and the lateral and medial pallium, whereas low levels were found in the remaining brain areas. It is interesting to note that the lowest binding densities of [3H] flunitrazepam plus an inverse binding pattern to that shown for the GABAA receptors were encountered in the two layers of the cerebellum. Consequently, when the binding densities of the two ligands were expressed as a ratio ([3H] MUS/[3H] FLU), we noticed that the highest ratio values were observed in the periventricular (17.1) and external (8) layers of the cerebellum, while a ratio of 1 was obtained in the external layer of the optic tectum. The heterogeneous binding relationship of these two GABAA molecular components encountered in key brain areas, such as the cerebellum and the optic tectum, of the frog Rana esculenta could provide interesting insights concerning the role of GABA in these brain sites.

ISSN:0006-8977    

DOI:10.1159/000114150

出版商:S. Karger AG    

年代:1993

数据来源: Karger

摘要:

Ocular morphology was examined in larval, juvenile and adult F. varium. There was a 26-fold increase in eye size from 0.28 mm in the smallest larva (5.0 mm in length) to a maximum diameter of 7.2 mm in a 110 mm long adult. Larval fish had pure cone retinae at hatching, however, putative rod precursor cells were also present. Juvenile and adult fish had a duplex retina with cones arranged in a square mosaic in which 4 equal double cones surrounded a central single cone. Hypertrophy of cone ellipsoids with increasing eye size resulted in maintenance of a closely packed array in fishes of all sizes. Theoretical sensitivity, assessed in terms of convergence of rods:bipolars, rod density, and photoreceptor outer segment length, increased during the juvenile phase but was constant across the adult size range. Angular density of cones increased with increasing eye size such that theoretical spacial acuity was poor in smallest fish (1°8′) and improved to an asymptotic value of about 9′ in adults. Behavioural acuity of a 1-day-old larva determined using the optokinetic response (29°), was very much poorer than histological estimates (1°8′). Behavioural acuity improved to 4°18′ at 14 days of age, compared to a theoretical value of 54′. An estimate of Matthiessen's ratio based on histological measurements suggests that the larval eye is initially strongly myopic, and grows into focus. Development of the retractor lends muscle was first apparent 7 days after hatching with the result that larval eyes are incapable of accommodative lens movements to correct for a refractive error. This apparent myopia is thought to account for at least part of the mismatch between theoretical and behavioural spatial acuity.

ISSN:0006-8977    

DOI:10.1159/000114151

出版商:S. Karger AG    

年代:1993

数据来源: Karger

摘要:

Nerve transection experiments combined with high-speed videography and electromyography were used to characterize a prey-type dependent hypoglossal feedback system which coordinates mouth opening and tongue protraction in the common leopard frog, Rana pipiens. When feeding on small prey, sensory feedback from the tongue through the hypoglossal nerve is necessary to trigger mouth opening. If sensory feedback is prevented from reaching the brain by transection of the hypoglossal nerve, then the mouth fails to open although the feeding behavior appears otherwise normal. However, when feeding on large prey, the mouth opens normally even after the hypoglossal nerve has been transected. Thus, peripheral feedback is not necessary to trigger mouth opening when feeding on large prey, and presumably a central coordination mechanism is used. In Rana pipiens, the evolution of a new tongue morphology and a new motor pattern for feeding on small prey has been accompanied by the evolution of a novel, peripheral mechanism for coordinating tongue protraction and mouth opening. However, the primitive motor pattern for feeding on large prey and the primitive coordinating mechanism have been retained. These results imply that the neural circuits producing the different motor patterns for large and small prey are anatomically distinct at some level in the central nervous system. If they are not anatomically distinct, then sensory feedback should be necessary to trigger mouth opening regardless of which motor pattern is being expressed. While the anatomy of these distinct pathways remains to be elucidated, these results suggest that novel neural circuits may in fact underlie different behaviors even when they seem, superficially, to be relatively similar.

ISSN:0006-8977    

DOI:10.1159/000114152

出版商:S. Karger AG    

年代:1993

数据来源: Karger